The state of the art includes optical sensors consisting of a plurality of photo-sensitive elements, or pixels, able to detect light signals and to transmit them, in the form of electric signals, to a calculator which processes them and obtains images from them which it transmits to display devices; the latter are then able to allow a user to see the images or information deriving therefrom.
Previously, such optical sensors were usually made using CCD technology (Charge-Coupled Device), which guarantees a very satisfactory image quality in the presence of a well controlled illumination, but are not able to operate optimally in the presence of a light with an intensity which is greatly differentiated inside the same scene, that is, with an input signal having high dynamics, up to 120 dB.
CCDs are also not very versatile from various points of view: they cannot easily be integrated with complex pilot circuits in a single silicon support or microchip, it is not possible to arbitrarily select one or more pixels inside the matrix sensor and it is possible to acquire the images only after having waited a latency time correlated to the integration time, or exposure, of each pixel to the light.
To overcome some of these shortcomings of CCDs, optical sensors have been developed based on the CMOS type silicon technology (Seger, Graf, Landgraf—“Vision Assistance in Scene with extreme Contrast”—IEEE Micro, vol. 13 page 50, February 1993), which offer a good result in very differentiated lighting conditions inside the same scene.
This result is obtained by means of a conversion on logarithmic scale of the signal inside the photo-sensitive element or pixel.
The logarithmic conversion, obtained for example by connecting one or more MOS type transistors in diode configuration to the photo-sensitive joint, as described in U.S. Pat. No. 5,608,204 or in JP 2001 250933 A, suffers in any case from the fundamental disadvantage that it supplies very weak output signals.
In JP'933, the gate of a third amplifying transistor T2 is connected to the gate of the first transistor T1 which makes the logarithmic conversion of the signal and not directly to the photo-sensitive element.
US 2001/0010548 does not disclose a circuit having two transistors connected in series and with the respective gate and drain short-circuited for obtaining the logarithmic conversion of the signal.
The prior art solutions tend to create difficulties in designing the signal amplifiers able to read the signals of the pixels and makes the sensors not very suitable for monolithic integration on a single microchip together with other circuits of a digital type, since they are excessively sensitive to the electric noise inevitably generated by such digital circuits.
The present Applicant has devised and embodied this invention to overcome the shortcomings of the state of the art and to obtain further advantages.